Fossil fuels are still one of the major energy sources nowadays. Example of fossil fuels such as coal, petroleum and natural gas are non-renewable natural resources which take thousands of years to form, and are costly to harvest. In addition, combustion of the fossil fuels generates large amount of carbon dioxide as well as undesirable air pollutants. To resolve this, clean and renewable energy resources such as solar energy which can be directly captured and converted to electrical energy by solar cells have been extensively researched and investigated. For example, photovoltaic (PV) technologies have been employed to fabricate different types of solar cells, such as crystalline silicon solar cells, amorphous silicon solar cells, dye-sensitized solar cells, copper indium gallium selenide (CIGS) solar cells and organic solar cells, etc.
Among the solar cells manufactured by various PV technologies, organic solar cells are known to have the advantages of low-cost and high process flexibility in production. Particularly, organic solar cells such as bulk heterojunction (BHJ) photovoltaic (PV) devices have been receiving considerable attention from both the academia and the industry as being promising alternative renewable energy sources due to their relatively low-cost, light-weight, ease of large-area fabrication by solution-process and compatibility to flexible substrate.